The present invention relates generally to an active system and method for mounting a vibrating component to a support structure for reducing the transmission of vibration and noise passing from the vibrating component to the support structure and, more particularly, to an active vibration and noise reduction system and method for use on a rotary wing aircraft.
Significant effort has been devoted to reducing the vibratory and acoustic loads on aircraft, particularly rotary wing aircraft such as helicopters, and the resulting vibration and noise that develops within the aircraft. A primary source of vibratory and acoustic loads in a helicopter is the main rotor system.
The main rotor system of a helicopter includes rotor blades mounted on a vertical shaft that projects from a transmission, often referred to as a gearbox. The gearbox comprises a number of gears which reduce the rotational speed of the helicopter""s engine to the much slower rotational speed of the main rotor blades. The gearbox has a plurality of mounting xe2x80x9cfeetxe2x80x9d which are connected directly to structure in the airframe which supports the gearbox.
The main rotor lift and driving torque produce reaction forces and moments on the gearbox. All of the lift and maneuvering loads are passed from the main rotor blades to the airframe through the mechanical connection between the gearbox feet and the airframe. The airframe structure which supports the gearbox is designed to react to these primary flight loads and safely and efficiently transmit the flight loads to the airframe.
In addition to the nearly static primary flight loads, the aircraft is also subjected to vibratory loads originating from the main rotor blades and acoustic loads generated by clashing of the main rotor transmission gears. The vibratory loads are strongest at a frequency equal to the rotational speed of the main rotor blades (P), which is generally between about 4 and about 5 Hz, multiplied by the number of rotor blades, typically 2 or 4. The product of the main rotor blades rotational speed and the number of blades is called the xe2x80x9cfundamentalxe2x80x9d. Tonals of decreasing vibratory strength occur at multiples of two, three and sometimes four of the fundamental. For example, for a 4 bladed rotor, this would correspond to 8P, 12P, and 16P.
The acoustic loads generated by the transmission gears are at a frequency that the gear teeth mesh with and contact each other, and are thus related to the type of construction and gear ratios used in the transmission. The acoustic loads also include a fundamental and tonals of decreasing strength at integer multiples of the fundamental. Typically, the noise generated by gear clashing is in the range of about 500 Hz to about 3 kHz.
The vibratory and acoustic loads produce vibrations and audible noise that are communicated directly to the helicopter airframe via the mechanical connection between the gearbox and the airframe. This mechanical connection becomes the xe2x80x9centry pointxe2x80x9d for the unwanted vibration and noise energy into the helicopter cabin. The vibrations and noise within the aircraft cabin cause discomfort to the passengers and crew. In addition, low frequency rotor vibrations are a primary cause of maintenance problems in helicopters.
In the past, xe2x80x9cpassivexe2x80x9d solutions have been tried for reducing the vibratory and acoustic loads on aircraft and the resulting vibration and noise that develops within the aircraft. For noise reduction, passive systems have employed broadband devices such as absorbing blankets or rubber mounts. However, broadband passive systems have generally proven to be heavy and, consequently, not structurally efficient for aircraft applications where weight is paramount. Additionally, broadband passive systems are not very effective at reducing low frequency vibration. A passive technique for reducing vibration involves the installation of narrowband, low frequency vibration absorbers around the aircraft that are tuned to the vibration frequency of interest, typically the fundamental. These narrowband, passive vibration reduction systems are effective, but the number of vibration tonals present in a helicopter may require a number of these systems which then adds significant weight. Additionally, narrowband passive systems work best when placed at ideal locations about the helicopter airframe, many of which may be occupied by other equipment.
More recently, xe2x80x9cactivexe2x80x9d vibration and noise reduction solutions are being employed since active systems have a much lower weight penalty and can be effective against both low frequency vibration and higher frequency noise. Active systems utilize sensors to monitor the status of the aircraft, or the vibration producing component, and a computer-based controller to command countermeasures to reduce the vibration and noise. The sensors are located throughout the aircraft and provide signals to the adaptive controller. The controller provides signals to a plurality of actuators that are located at strategic places within the aircraft. The actuators produce controlled forces or displacements which attempt to minimize vibration and noise at the sensed locations.
Low frequency motion (i.e., vibration) behaves according to rigid body rules and structural models can be used to accurately predict the nature and magnitude of the motion. Since low frequency motion is easily modeled, its negative effects can be cancelled with an active system of moderate complexity. High frequency motion (i.e., noise) at the transmission gear clash frequencies does not obey the rigid body rules present at low vibration frequencies. The use of riveted airframes in combination with the complex mode shapes present at high frequencies make structural models much less accurate. As a result, active systems for high frequency energy reduction become more complex, requiring large numbers of actuators and sensors to counter the more complex modal behavior of the airframe structure.
Some active systems utilize hydraulic actuation systems and hydraulic actuators to reduce vibration and noise. The hydraulic actuation system is preferred since the hydraulic system provides the necessary control bandwidth and authority to accommodate the frequencies and high loads typically experienced in an aircraft such as a helicopter. Additionally, aircraft typically have hydraulic power sources with spare capacity which can be utilized or augmented.
Two methods of actuator placement are frequently used in active systems: (1) distribute the actuators over the airframe, or (2) co-locate the actuators at, or near, the vibration or noise entry point. The co-location approach places the actuators at or near the structural interface between the transmission and airframe, stopping vibration and noise near the entry point before it is able to spread out into the aircraft. This has the advantage of reducing the number of actuators and the complexity of the control system. Active systems using this approach employ actuators mounted in parallel or in series with the entry point to counteract the vibration and noise.
The distributed actuator approach requires a large number of actuators for controlling noise due to the high frequencies, and their associated short spatial wavelength. The large number of actuators can drive up weight and add significantly to control system complexity. One distributed actuator active noise reduction system for use in a helicopter application uses more than 20 actuators to control transmission noise. Distributed actuators for low frequency vibration will be less numerous and are effective at reducing vibration at the sensor locations, but can drive vibration at other areas of the aircraft to levels exceeding those already present.
The parallel actuator approach is effective for low frequency vibration but can produce counteracting forces in the supporting structural elements which can exceed the design limit of the elements and lead to premature failure. Additionally, the parallel approach is not effective at reducing noise because the parallel actuator provide a direct path for noise entry.
The series approach is the most effective in reducing cabin vibration and avoids the introduction of unwanted vibrations. This approach would use actuators mounted in series between the transmission gearbox feet and airframe support structure. In this approach, the gearbox and airframe are isolated from each other connected only by actuators. The gearbox would vibrate in its own inertial frame separately from the airframe""s inertial frame, isolating the gearbox and airframe in a dynamic sense. This approach interrupts the transmission of vibratory and acoustic energy through the principal entry point thereby preventing vibration and noise from entering the airframe. For this approach to be effective, the vibration and noise isolation system must support the large, static primary flight loads along an axis also requiring dynamic isolation. This system must maintain the average static position of the transmission relative to the airframe for proper operation of the other helicopter systems, particularly the helicopter engines that couple into the transmission. However, in the series approach, the high frequencies associated with noise lead to complex motions at the entry point which, if fully addressed, may lead to large and heavy actuators to actively control all degrees of freedom at each entry point.
For the foregoing reasons, there is a need for a new system for reducing both vibration and noise in aircraft applications, and particularly helicopters. Preferably, the new vibration reduction system will rely on an active solution for isolating a vibratory load source, such as the main rotor system of the helicopter, and preventing the low frequency vibration generated by the main rotor system from being transmitted to the airframe. The system should efficiently pass the primary flight loads while maintaining the average static position of the transmission relative to the airframe. The system should utilize a hydraulic actuation system and actuators, taking advantage of the hydraulic power capacity on the aircraft.
The new system should also include a device and method for reducing noise. In a helicopter application, this new device should address the high frequency noise generated in a helicopter by the clashing of the transmission gear teeth. Ideally, the new device should function in cooperation with the other elements of the system. The new device should also take advantage of the isolation properties of the vibration reduction system to effectively reduce transmission noise without significantly increasing weight and minimizing the complexity of the overall vibration and noise reduction system.
It is an object of the present invention to provide a system for simultaneously reducing both vibration and noise in aircraft applications, and particularly helicopters.
Another object of the present invention is to provide an active device and system for isolating the main rotor system of a helicopter from the airframe for preventing the low frequency vibration generated by the main rotor system from being transmitted to the airframe.
A further object of the present invention is to provide an active vibration reduction system for passing the primary flight loads of the helicopter from the main rotor system to the airframe while maintaining the average static position of the transmission relative to the airframe.
According to the present invention, an active mount is provided for use in a rotary wing aircraft, the active mount comprising first and second linear hydraulic actuators each having a principal axis and adapted to be disposed between each of the gearbox and airframe mounting locations for mechanically suspending the airframe from the gearbox. The length of the actuators is variable along the principal axis for providing relative movement between the airframe and the gearbox. The principal axes of the actuators are adapted to lie in the directional planes of the primary forces necessary for supporting the airframe and acting on the transmission gearbox mounting locations for providing movement of the gearbox relative to the airframe in the planes at a frequency for reducing the transfer of vibration through the active mount to the airframe. In one embodiment, the active mount comprises first and second rigid members and the actuators comprise a piston slidably disposed in a cylinder for linear reciprocal movement along the principal axis. The first rigid member is connected to the airframe and the second rigid member is connected to the gearbox. One end of each hydraulic actuator is attached to the first rigid member and a piston rod extending from the other end of the actuator is connected to the second rigid member.
Further in accordance with the present invention, a system is provided for reducing vibration in a rotary wing aircraft, including the active mount, and further comprising hydraulic actuation means for supplying pressurized hydraulic fluid to the actuators. The hydraulic actuation means includes a source of pressurized hydraulic fluid and an electro-hydraulic valve hydraulically connected between the source and the hydraulic actuators. Means for sensing parameters related to vibration from the airframe produce a signal output representative of the sensed parameter. Means operatively connected to the valve and the sensing means receive the signals produced by the sensing means and generate output control signals to the valve responsive to the sensing means signals. The sensor signal receiving and control signal generating means may comprise a computer for applying a programmed control algorithm to the sensor signals for generating the valve control signals. The output control signals operate the valve for selectively supplying a flow of pressurized hydraulic fluid from the source to the actuators. The length of the actuators are thus varied in the planes of the primary supporting forces thereby moving the gearbox in the planes relative to the airframe at a frequency for reducing the transfer of vibration through the active mount to the airframe.
Still further in accordance with the present invention, a method is provided for reducing vibration in a rotary wing aircraft, the method comprising the steps of disposing the active mount between each point of contact of the gearbox and airframe for mechanically suspending the airframe from the gearbox, sensing parameters related to vibration or noise from the airframe and producing a signal output representative of the sensed parameter. The method of the present invention further includes the steps of processing the signals produced by the sensor, generating an output control signal to the valve responsive to the sensed signal for operating the valve to selectively supply a flow of pressurized hydraulic fluid from the source to the actuators, and moving the gearbox relative to the airframe in the planes of the primary supporting forces acting on the transmission gearbox mounting locations by varying the length of the actuators at a frequency for reducing the transfer of vibration through the active mount to the airframe.
Also in accordance with the invention, a rotary wing aircraft is provided including the active vibration reduction system and method.
The present invention features two hydraulic actuators at each mounting location of the transmission gearbox to the airframe. The principal axis of the first and second actuators may be substantially vertical and substantially horizontal, respectively. Alternatively, the principal axis of the second actuator may also form an acute angle with the longitudinal axis of the airframe. Similarly, the principal axis of the first actuator may form an acute angle with the vertical longitudinal and vertical lateral planes of the airframe, or the principal axis of the second actuator may form an acute angle with the vertical longitudinal and horizontal longitudinal planes of the airframe. The object is to provide actuators whose principal active axes lie in the directional planes of the primary forces necessary for supporting the airframe and acting on the transmission gearbox mounting locations for providing movement of the gearbox relative to the airframe in the planes.
In a helicopter, the active mounts may comprise four active mounts with two such mounts being forward active mounts located forward of the rotor and on opposite lateral sides thereof and the two remaining mounts being after active mounts located aft of the rotor axis of rotation and on opposite lateral sides thereof. In one embodiment, a first valve controls the delivery of fluid to the first forward active mounts and a second valve controls the delivery of fluid to the second forward active mounts.
Another feature of the present invention is the hydraulic actuation means which provides a quasi-steady pressure to each actuator to support the applied quasi-steady flight and maneuvering loads and for maintaining the transmission gearbox in a steady, static position relative to the airframe at maneuvering frequencies.
The active vibration reduction system and method adopts the series approach for active mount placement with the actuators mounted in series between the transmission gearbox feet and airframe support structure for reducing vibration on board a rotary wing aircraft. The gearbox and airframe are isolated, thereby interrupting the transmission of vibratory and acoustic energy through the active mount to the airframe. The active mount of the present invention supports the static primary flight loads while maintaining the average static position of the transmission relative to the airframe. Moreover, the system requires only two actuators per mount which significantly reduces the complexity and weight of the system. A passive noise isolator incorporated into the system serves to reduce high frequency noise generated by the transmission gear teeth.
The foregoing and other features and advantages of the present invention will become more apparent in light of the following detailed description of the preferred embodiments thereof, as illustrated in the accompanying figures. As will be realized, the invention is capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and the description are to be regarded as illustrative in nature, and not as restrictive.